The potential of filtration for the purification of waste liquids and waste gases in biofilters has been recognized due to its efficiency and low cost. Typically, an organic support in a biofilter directly retains or fixes different types of pollutants and can act as a support for various types of microorganisms capable of degrading the pollutants. The constituents of organic supports, lignin and organic acids in particular, possess numerous functional polar groups conferring them a good absorption capacity for organic molecules as well as transition metals.
One of the organic supports that are typically used is peat. Peat is a polar and highly porous material, in addition to being inexpensive and easy to use. It is capable of adsorbing many undesirable elements and reducing their level from the environment. It removes pollutants and microbial activity from waste liquids by adsorption and absorption reactions. Peat is also effective at removing suspended solids, nutrients, heavy metals, organic matter, oils, odors and bacteria.
Several designs of biofilters to treat domestic waste liquids have been developed in the past. They are usually composed of four layers, namely a bottom layer of coarse gravel, a layer of sand, a layer of peat and an upper layer of grass. The two bottom layers act as a support to retain the peat bed. These biofilters are generally used for the treatment of small volumes of waste liquids. A biofilter exhibiting higher filtering capacity for the purification of agricultural, municipal or industrial waste water is disclosed in U.S. Pat. No. 6,100,081 to Buelna and naming the same assignee as the present application. That biofilter makes use of a filtering media formed by a succession of layers including a filtering carrier material such as peat and a structural carrier material such as wood chips in a sufficient amount to prevent compaction of the filtering material and channels formation, whereby the waste water and an O2-containing gas can flow to perform pollutant degradation by microorganisms. More particularly, the media is characterized by increasing the filtration capacity in a direction of flow of the waste water to prevent clogging of the filtering material.
Some other prior art biofilters make use of inert granular filtering medias, such as the biological reactor adapted to purify sewage or water disclosed in U.S. Pat. No. 4,997,568 to Vandervelde et al. That biological reactor includes an inverted conic, water impermeable chamber receiving at the apex thereof contaminated water or sewage that then flows upwardly and radially through layers of filtering media providing progressive treatment by reduced pore size and increased surface area as the flow proceed to the periphery. The layers of media are constructed and arranged to provide passive ambient air contact at an upper surface of the layers and to provide an extended residence time to the water or sewage being treated.
Another type of biological aerated filter using inert filtering media is disclosed in U.S. Pat. No. 5,800,709 to Smith. This filter is in the form of a treatment vessel including a high voidage, coarse granular media bed lying on a grid and located above a low voidage, fine granular media bed, which media beds are separated by an intermediate section containing a surplus treated water outlet for recycling and a washout collector. An O2-containing gas is supplied, which gas may flow upwards through all of the media beds, or be completely removed from the intermediate section to prevent O2 from the lower aerated media from passing to the upper flooded anoxic media bed.
Although biofilters have been used with success over the years, there is still a need to improve them and solve some of their drawbacks, especially for treatment of highly polluted liquids. For instance, low maintenance, non-backwash type biofilters currently in use are not designed to alleviate the formation of a compact microbial layer at the surface of the filtering material over long period of operation, typically more than one year. This microbial layer generally leads to the formation of a clogging layer on the filtering material inside the biofilter, thus promoting the formation of preferential gas pathways where aeration is used. This significantly lowers the life-span and efficiency of the biofilter.